A. Field of the Invention
This invention relates to antennas for high frequency broadcasting and for communication systems.
B. Prior Art
When considering antennas for high frequency broadcasting and communication systems there are a few basic types which have been in use for many years. Normally a directional antenna is desired since the target area often includes a single language speaking country or the circuit is a point-to-point communications path. The directional antenna conserves energy and concentrates the total transmitter power in the desired direction or area. The width of the beam and the resulting power gain are closely related in the following way:
______________________________________ Beam width 90 60 30 10 Degrees Power gain 30 45 90 275 Ratio-to-one Gain 15 17 20 24 dBI ______________________________________
These are nominal values and vary somewhat with the vertical beam width and magnitude of the side lobes.
The most used antenna for international broadcasting is the 16 element curtain antenna stacked four high and four wide using half wave dipoles in front of a reflector screen. It is a good antenna to use as a standard for comparison of all other types. The principle concern is how many antennas are required to cover the frequency range from approximately 5 to 30 MHz and consequently the total cost of the system.
The bandwidth of the curtain antenna is limited both by its impedance bandwidth and its radiation pattern bandwidth. Classic curtain antennas, such as the Voice of America Type IIC, are used over two adjacent international bands or a frequency ratio of 1.3:1. With improved dipole design, the ratio can be increased to 1.5:1. Beyond that point, there is no need for further improvements to the impedance bandwidth as the pattern bandwidth is restricted to that range. As will be illustrated further in the figures, if a beam of a curtain antenna is set to a useful angle of 10.degree. or 12.degree. at the low frequency end of its range, then as the frequency is increased the pattern becomes multilobed, it moves downward and its performance in gain and multipath interference deteriorates rapidly. Nothing further can be done to improve the pattern performance since it is a function of the physical height of the antenna array above the ground. These facts are all well documented in classical antenna literature. For a 1.5:1 range, a minimum of four curtains are required to cover from 5 to 25 MHz and preferably five are used for good performance.
Another type of antenna that has been used is the Rhombic. The impedance bandwidth of a terminated Rhombic is unlimited. Its pattern bandwith, however, has the same limitations as curtain antenna arrays and for the same reasons. Very long Rhombics tend to control the over-hanging lobes a little better than curtains and have been used over a 2:1 frequency ratio for economy. If the leg lengths are in excess of 5 wavelengths, it is not necessary to terminate the far end of the Rhombic with a load resistor since the reflection coefficient is largely radiated before it returns to the input terminals and the impedance excursions are consequently small. Four large Rhombics can be used to cover the frequency range with good performance but require a considerable amount of real estate.
The log periodic antenna, which has also been used, is like the Rhombic because it can be designed for a low impedance bandwidth over an unlimited frequency range and thus its impedance bandwidth is unlimited. When we consider pattern bandwidth, several types of log periodic antennas must be described. For example, a horizontal log periodic antenna has exactly the same pattern bandwidth limitation as the Curtain and Rhombic antennas and for the same reasons. It is possible to design a steeply sloping horizontal log periodic antenna wherein each of its elements has constant electrical height above the ground. This will produce a constant pattern over any frequency range, but the pattern has one high angle lobe good for short distance transmission only. It cannot be used for long range work.
Vertically polarized log periodic antennas have been in use to overcome the pattern deficiency of horizontal log periodics. However, while the pattern bandwidth is unlimited, it is not necessarily what is desired unless a very extensive ground system is used for thousands of feet out in front of the array. Low angle summation of the incident and ground reflected waves, as in-phase vectors, is based upon the assumption of a perfectly conductive earth. That is approximately true at medium and low frequencies, but at 5 MHz and above, it is far from true, as will be described in connection with the figures herebelow.
The reflection coefficient in this type of antenna is 180.degree. out of phase at low angles for average soil conductivity from 5 to 30 MHz. Only above 20.degree. does it become reasonably close to being in-phase, however at such high angles its magnitude has decreased to a small fraction of its value before the reflection due to dissipation in the soil. Thus, there is little or no radiation at low angles and at high angles there is up to 3 dB loss in the system, which means that one half the power is wasted in the earth. The only solution offered to this problem to date is to provide a ground screen out through the first fresnel zone which, for low angles is quite extensive. This requires a lot of copper, labor and real estate.